Determination of total lead and lead species according to their lability in coastal seawater by Chelex-100 titration and electrothermal-atomic absorption spectrometry

Abstract

A speciation method based on Chelex-100 titration in combination with electrothermal-atomic absorption spectrometry has been developed to study the distribution of Pb among different fractions in coastal seawater. The method allows an accurate determination of total Pb, thereby avoiding the effect of the saline matrix. Additionally, the distribution patterns of Pb among species with different lability can be established in seawater as a function of pH. Four Pb fractions have been obtained in a model seawater and two seawater samples from Ria de Vigo (NE Atlantic ocean), i.e. free Pb ion, inorganic Pb fraction, organic Pb fraction and inert Pb fraction. The inert and labile organic Pb fractions dominate at natural pH (≈8).

Complexation mechanisms of Pb were investigated from the sorption curves of lead on Chelex-100 according to the Gibbs–Donnan model. Three complexes were formed, PbL, PbL₂ and Pb(HL)₂ with the following intrinsic complexation constants logβ_(1np) = ?3.25, ?10.10 and ?1.98, respectively. The formation of the complex PbL is negligible in all range of pH studied, while the complex Pb(HL)₂ predominates up to pH 4 and PbL₂ from pH 4.